Processing of Monolayer Materials Via Interfacial Reactions

ABSTRACT

A method of forming and processing of graphene is disclosed based on exposure and selective intercalation of the partially graphene-covered metal substrate with atomic or molecular intercalation species such as oxygen (O 2 ) and nitrogen oxide (NO 2 ). The process of intercalation lifts the strong metal-carbon coupling and restores the characteristic Dirac behavior of isolated monolayer graphene. The interface of graphene with metals or metal-decorated substrates also provides for controlled chemical reactions based on novel functionality of the confined space between a metal surface and a graphene sheet.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a continuation application of copending U.S. patentapplication Ser. No. 13/468,592, filed on May 10, 2012, which claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Application No.61/484,752 filed on May 11, 2011, the content of which is incorporatedherein in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under contract numberDE-AC02-98CH10886, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

I. FIELD

This relates generally to the processing of monolayer graphene-basedmaterials. In particular, this relates to the processing of large-area,structurally perfect monolayer graphene domains on metal ormetal-decorated substrates via interfacial reactions. This furtherrelates to the utilization of graphene layers in complex chemicalreactions in a confined space between the graphene layer and the metalor metal-decorated substrate. This also relates to the utilization ofthe graphene layer(s) in electronic devices, such as sensors, catalysts,or for mechanical purposes.

II. BACKGROUND

Monolayer materials, such as graphene, are materials with greatpotential for electronics and other future carbon-based devicearchitectures. Graphene is the two-dimensional (2D) form of crystallinecarbon. It is a single atomic sheet of sp²-bonded carbon arranged in ahoneycomb lattice extending in a single plane. As illustrated in FIGS.1A-1D, graphene is the building block for the entire family of graphiticmaterials. For instance, graphene formed into a ball (see FIG. 1B)results in a carbon fullerene (buckyball); formed into a tube (see FIG.1C) results in a carbon nanotube; and stacked at least ten layers high(see FIG. 1D), the graphene transforms into bulk graphite.

In fact, by stacking more and more graphene layers on top of each other,the material's properties change dramatically. A single layer ofgraphene exhibits a quantum staircase in Hall conductivity and ballistictransport, i.e., its charge carriers behave as massless Dirac fermions:charge carriers in the single layer can travel thousands of interatomicdistances without scattering. Nanoscale ribbons of graphene exhibitquantum confinement, and the capability for single-molecule gasdetection. Graphene's physical properties are equally impressive.Measurements probing the intrinsic strength of a sheet of graphenereveal that it is the strongest known material. At two layers thick,graphene is still a zero-gap semiconductor exhibiting the quantum Halleffect. But, unlike single-layer graphene, double-layer graphene lacks afirst “step” in the quantum staircase. For three or more graphenelayers, however, the electronic properties begin to diverge, ultimatelyapproaching the 3D limit of bulk carbon at about ten layers in thicknessand more appropriately referred to as graphite.

One distinct advantage of graphene lies in its 2D nature, so that thedrive current of a graphene device, in principle, can be easily scaledup by increasing the device channel width. This width scaling capabilityof graphene is of great significance for realizing high-frequencygraphene devices with sufficient drive current for large circuits andassociated measurements. Furthermore, the planar graphene allows for thefabrication of graphene devices and integrated circuits utilizingwell-established planar processes in the semiconductor industry. Areview of graphene is provided, for example, by A. K. Geim, et al. in“The Rise of Graphene,” Nature Materials 6, 183 (2007) and in “Graphene:Exploring Carbon Flatland,” Physics Today, 60, p. 35 (2007) each ofwhich, along with the references cited therein, is incorporated byreference in its entirety as if fully set forth in this specification.

These remarkable properties make graphene suitable for a wide variety ofapplications. Potential applications in electronics include use ofgraphene as a new channel material for field-effect transistors (FETs)and as a conductive sheet in the fabrication of single-electrontransistor (SET) circuitry. Another potential application isgraphene-based composite materials in which a graphene powder isdispersed within a polymer matrix. Graphene powder may also findapplications in batteries, as field emitters in plasma displays, or as acatalyst due to its extraordinarily high surface area. Single graphenesheets have exceptionally low-noise electronic characteristics, therebylending the possibility of their use as probes capable of detectingminuscule changes in external charge, magnetic fields, or mechanicalstrain.

Despite the extraordinary potential of graphene, realization ofpractical applications which exploit its unique properties requires thedevelopment of reliable methods for fabricating large-area,single-crystal, and defect-free graphene domains. Recent attempts toproduce monolayer and/or few-layer graphene have involved, for example,mechanical exfoliation of graphite crystals, thermal decomposition ofsilicon carbide (SiC) at elevated temperatures, reduction of grapheneoxide in hydrazine, and epitaxial growth on transition metal surfaces.However, it continues to be a challenge to efficiently and reproduciblyform large (>100 μm) single-crystal domains in quantities sufficient forlarge-scale fabrication.

For instance, chemical exfoliation involves inserting (“intercalating”)molecules into bulk graphite in order to separate the crystalline planesinto individual graphene layers. The benefit of this technique is itsfacile chemical approach. The problem, however, is that even after theintercalating molecules are removed from the mixture, the resultantcarbon compounds are present in a “sludge,” which contains bothrestacked and scrolled graphene sheets. (See M. S. Dresselhaus & G.Dresselhaus, Adv. Phys., 51, 1-186, (2002), incorporated herein byreference in its entirety.) Chemical epitaxy, on the other hand, offersthe solution to graphene's large-scale integration challenge. In oneversion of the method, graphene is grown via chemical vapor deposition(CVD) of hydrocarbons deposited on a metal substrate. But, the presence(or remaining residue) of the metal substrate used in the CVD methodmight not be compatible with electronic fabrication. In contrast to theCVD method, the thermal decomposition method begins with asemiconducting SiC substrate, which is heated to over 1200° C. until thesilicon begins to sublime, at which point the remaining carbon on top ofthe substrate nucleates into graphitic film. The resultant graphene/SiCsample can then be mounted on a silicon substrate for deviceintegration. This thermal decomposition method can achieve few-layergraphene that exhibits high-mobility charge transport. This method,however, requires high-temperature vacuum processing. Consequently, theformation of graphene domains with uniform thicknesses and length scalessufficient for practical applications remains a challenge. (See C.Berger et al., J. Phys. Chem. B 108, pp. 19912-19916, (2004),incorporated herein by reference in its entirety.) One approach toepitaxially grow the graphene on the ruthenium (Ru) transition metalthat avoids the shortcomings noted above is described in U.S. Pat. Pub.No. 2010/0255984 to Sutter et al.

However, while epitaxial growth on transition metal surfaces is key torealizing large-scale graphene growth, forming conventional andspin-polarizing device contacts, and accessing functionalities such asmagnetism and superconductivity, as well as having importantimplications for transition-metal surface chemistry and catalysis in thepresence of graphitic carbon, the method also results in a stronginterfacial interaction of transition metal with graphene thatsuppresses the characteristic linear π bands of its electronicstructure. This suppression hinders the rise of the high-mobilitymassless Dirac quasi-particles.

Efforts to change the graphene-transition metal interaction have largelyfocused on intercalation of metal atoms and, recently, hydrogen (Forexample, see Varykhalov, A. et al., Phys. Rev. Lett., 101, p. 157601(2008); Oshima, C. and Nagashima, A., J. Phys: Condens. Matter, 9, pp.1-20 (1997); Nagashima, A. et al., Phys. Rev. B, 50, pp. 17487-17495(1994); and Riedl, C. et al., Phys. Rev. Lett., 103 p. 246804 (2009);each incorporated herein by reference in its entirety.)

Thus, despite the extraordinary potential of graphene, realization ofpractical applications that exploit its unique properties requires thedevelopment of reliable methods for fabricating large-area,single-crystal, and defect-free graphene domains that can be effectivelylifted off the metal substrate despite a strong metal-carbon couplingand thereby restore the characteristic linear π bands that give rise tohigh-mobility massless Dirac quasi-particles in the monolayer graphene.

SUMMARY

The complex behavior induced by atomic or molecular intercalationspecies exposure of partially graphene-covered metal has importantimplications for the processing of graphene for device applications aswell as for transition metal surface chemistry and catalysis in thepresence of graphitic carbon. Growth on transition metals has become oneof the leading contenders for large-scale graphene synthesis. It iscommonly accepted that for applications in electronics, the grapheneneeds to be transferred from the growth substrate to an insulatingsupport. Thus, a novel method of forming and processing of graphene isprovided based on exposure and selective intercalation of the partiallygraphene-covered metal substrate with atomic or molecular intercalationspecies such as oxygen (O₂) and/or nitrogen oxide (NO₂). In oneembodiment, the process of intercalation lifts the strong metal-carboncoupling and restores the characteristic Dirac behavior of isolatedmonolayer graphene.

A method of growing and processing graphene includes a step ofepitaxially depositing a layer of carbon based material on a surface ofa metal to form a layer of graphene as described in U.S. Pat. Pub. No.2010/0255984 to Sutter et al., which is incorporated herein by referencein its entirety. In this embodiment, the metal preferably includes, butis not limited to, any transition metal or alloy that exhibits a largechange in C solubility with changing temperature. For example, thetransition metal may be selected from ruthenium (Ru), nickel (Ni),platinum (Pt), iridium (Ir), or copper (Cu), while ruthenium's (0001)crystal surface is preferred. A detailed description of the process forpreparing a monolayer graphene on the surface of the Ru(0001) isdescribed in Sutter, P. W., Flege, J. I., and Sutter, E. A., “Epitaxialgraphene on ruthenium,” Nat. Mater., 7, pp. 406-411 (2008) (hereinafter“Sutter 2008”), which is incorporated herein by reference in itsentirety. In another embodiment a surface template for graphene growthmay be provided by a suitable transition metal foil or a transitionmetal layer formed on a supporting substrate.

The method of growing and processing the graphene further includes stepsof exposing the partially covered surface of the metal, e.g., Ru(0001),to an ambient gas, e.g., oxygen (O₂) or nitrogen oxide (NO₂), and tuningthe graphene-metal interaction by interfacial reaction of the ambientgas with the surface of the metal. The step of tuning the graphene-metalinterface can be achieved by regulating surface exposure of thetransition metal to ambient gas. In one embodiment, the ambient gasmolecules adsorb on the surface of the metal at temperatures below 400°C., and preferably between 20° C. and 400° C., which in turn decouplesthe graphene from the metal. In this embodiment, the ambient gas, suchas oxygen, does not etch the graphene but selectively adsorbs on themetal surface beneath the graphene sheet. The complete intercalation ofmacroscopic domains that are tens of micrometers in size decouples thegraphene and restores the linear π bands of its electronic structure.The graphene sheet is not merely a passive spectator in this process,but its presence affects the metal-adsorbate interaction. In thisembodiment, the intrinsic bonding strength of an adsorbate on the cleanmetal surface can be modified by partially covering the metal surfacewith a graphene sheet. In another embodiment, the intercalation can bereversed by raising the system to a temperature above 400° C.,preferably above 450° C. and below the melting temperature of thetransition metal, e.g., Ru˜2334° C.

Another aspect is the novel functionality of the confined space betweenthe metal surface and the graphene sheet that is conducive to controlledchemical reactions. The functionality stems from a steric hindrance(˜3.3 Å) between the graphene sheet and the surface of the metal.Specifically, the steric hindrance limits access of undesirable atomicand molecular species, especially larger molecules, and controls theorientation of desirable molecular species, which in turn has an effecton the reaction parameters, such as adsorption energies and can inducethe selective bonding and reaction of properly oriented molecularspecies. In this embodiment, it is possible to perform controlledchemical reactions at the interface with graphene that may be exploitedto tune chemical and catalytic reactions or to tune graphene'selectronic structure for the fabrication of device elements. In someembodiments construction of interfacial layers may occur byintercalation of reacting species at the interface between graphene anda substrate.

The method of processing graphene further includes providing a graphenelayer in which graphene interacts with a surface of a metal substrateunder the graphene layer and, the surface of the metal substrate isexposed to a basic aqueous solution. The graphene-substrate interactionis tuned by interfacial reaction of the basic aqueous solution on thesurface of the metal surface, and the basic aqueous solution isintercalated between the graphene layer and the surface of the metalsubstrate as part of a working electrode in an electrochemical cell. Thegraphene-substrate interaction is modified or the graphene is decoupledfrom the metal substrate as a result of the basic aqueous solutionintercalation.

These and other characteristics will become more apparent from thefollowing description and illustrative embodiments which are describedin detail with reference to the accompanying drawings. Similar elementsin each figure are designated by like reference numbers and, hence,subsequent detailed descriptions thereof may be omitted for brevity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show (A) a 2D graphene sheet that can be formed into (B) 0Dbuckyballs, e.g., C₆₀ fullerene, (C) 1D nanotubes, or (D) stacked 3Dgraphite.

FIG. 2A is a sequence of LEEM images obtained during high-temperature O₂exposure, showing oxygen etching of a graphene domain (P=5×10⁻⁷ Torr;T=550° C.).

FIG. 2B is a time-dependent image intensity [I(x, t)] map along the linemarked in FIG. 2A.

FIG. 2C is a sequence of LEEM images obtained during low-temperature O₂exposure, giving rise to oxygen intercalation and selective oxidation ofthe Ru surface beneath graphene (P=5×10⁻⁷ Torr; T=340° C.), leaving thegraphene intact.

FIG. 2D is a time-dependent image intensity [I(x, t)] map along the linemarked in FIG. 2C.

FIG. 2E is a sequence of LEEM images obtained during low-temperature NO₂exposure (P=2×10⁻⁷ Torr; T=340° C.).

FIG. 2F is a time-dependent image intensity [I(x, t)] map along the linemarked in FIG. 2E.

FIG. 3A is a micrometer-sized spot angle-resolved photoelectronspectroscopy (micro-ARPES) map of the band structure of as-grownmonolayer graphene on Ru(0001), reflecting the strong coupling betweengraphene and Ru by hybridization of graphene's electronic structure withmetal d states.

FIG. 3B is a schematic of the corrugated Moiré structure of graphene onRu(0001) with alternating strong and weak coupling between graphene andRu corresponding to FIG. 3A.

FIG. 3C is a micro-ARPES map of the band structure of the graphenesample of FIG. 3A after exposure to O₂, showing the restoration oflinear π bands crossing the Fermi energy (E_(F)) and hole doping of thegraphene with a charge-neutrality point 0.5 eV above E_(F).

FIG. 3D is a schematic of the decoupled, planar graphene sheet over anordered Ru(0001)-(2×1)-O structure.

FIG. 4A is an Arrhenius plot showing different activation energies forintercalation (0.38 eV) and etching (1.1 eV).

FIG. 4B shows derived net reaction rates for etching and intercalation,illustrating the branching into two distinct regimes at low and hightemperatures.

FIG. 5A shows a schematic illustration of chemisorption on the clean Rusurface

FIG. 5B shows a schematic illustration of saturation of the dissociativeadsorption at an oxygen coverage of 0.5 ML in an ordered (2×1)-Ostructure.

FIG. 5C shows a schematic illustration of molecular intercalation of O₂beneath monolayer graphene on Ru, leading to simultaneous graphene-metaldecoupling and formation of the (2×1)-O saturation structure.

FIG. 6 is a low-bias scanning tunneling microscopy image of the boundarybetween as-grown and oxygen-intercalated graphene on Ru(0001),illustrating the elimination of the corrugated moiré structure to form aplanar, graphene layer that is decoupled from the metal.

FIG. 7A is a low-energy electron diffraction (LEED) pattern (155 eV) ofas-grown monolayer graphene on Ru(0001).

FIG. 7B shows a LEED intensity map as a function of in-plane wavevector,k₁ on as-grown monolayer graphene on Ru(0001)

FIG. 7C is a LEED pattern (155 eV) of oxygen- (O₂—) intercalatedmonolayer graphene on Ru(0001).

FIG. 7D is a (k∥, E)-dependent diffraction intensity map forO₂-intercalated monolayer graphene, showing the formation of an ordered(2×1)-O superstructure.

FIG. 8A is a UHV scanning electron microscopy image showing lens-shapedmonolayer graphene domains on Ru(0001) after full intercalation byexposure to nitrogen dioxide (NO₂) at 300° C.

FIG. 8B is a graph of UHV nano-Auger electron spectra obtained at pointsnear the center and the periphery of a graphene domain, marked in FIG.8A.

FIG. 8C is a graph of nitrogen and oxygen KLL Auger lines, showing bothN and O at the center of the graphene domain (upper spectrum), but onlyO near the periphery (lower spectrum).

FIG. 9A is a series of LEEM images of an initially fully O₂-intercalatedmonolayer graphene domain, at different stages of annealing to a peaktemperature of ˜400° C. (temperature profile shown in FIG. 9C).

FIG. 9B is a time-dependent image intensity [I(x, t)] map along the linemarked in FIG. 9A.

FIG. 9C is a graph showing the annealing temperature profile andtime-dependent LEEM image contrast at the points on the free metalsurface and within the graphene domain, marked in FIG. 9A.

FIG. 10 shows a schematic illustration of size control of chemicalreactions at the interface between graphene and a substrate.

FIG. 11 shows a schematic illustration of interfacial materialssynthesis at the interface between graphene and a substrate.

DETAILED DESCRIPTION

A method of processing graphene is provided by combining atomic andmolecular intercalation of different species, e.g., Si, O₂, and NO₂, inorder to liberate a graphene sheet from the strong metal-carbon couplingand, thereby, restore the characteristic Dirac behavior of isolatedgraphene. The graphene includes monolayer graphene, or has relatedcharge-carrier characteristics of bilayer graphene, few-layer grapheneor multilayer graphene.

Such intercalation may generate thin gate insulators beneath grapheneand, following suitable lithographic patterning, allows utilization ofthe underlying metal as source, drain, and gate electrodes in afield-effect device. It is to be understood, however, that those skilledin the art may develop other combinatorial, structural, and functionalmodifications without significantly departing from the scope of thisdisclosure.

A method of processing a monolayer graphene includes the steps ofgrowing graphene by epitaxially depositing a layer of carbon basedmaterial on a surface of a metal substrate, exposing the surface of themetal substrate under the graphene layer to an ambient gas or otheratomic or molecular species, and tuning the graphene-metal substrateinteraction by interfacial reaction of the ambient gas or other atomicor molecular species on the surface of the metal substrate. The ambientgas is an atomic gas, a gas of diatomic or larger molecules, or a gas ofmolecules that break down into atoms or smaller (diatomic or larger)molecules between the graphene layer and the metal substrate surface.Examples of ambient gases include oxygen (O₂), nitrogen oxide (NO₂),nitrogen (N₂), hydrogen (H₂), chlorine (Cl), fluorine (F), bromine (Br),iodine (I), and ammonia (NH₃). Other species appropriate forintercalation include silicon (Si), boron (B), aluminum (Al), zinc (Zn),chromium (Cr), titanium (Ti), zirconium (Zr), hafnium (Hf), scandium(Sc), yttrium (Y), magnesium (Mg), calcium (Ca), strontium (Sr), barium(Ba), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sa), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), andytterbium (Yb).

In the step of growing graphene epitaxially, the metal substrate needsto provide a growth template for the graphene. In one embodiment, themetal substrate includes, but is not limited to, any transition metal oralloy that exhibits a large change in C solubility with changingtemperature. Preferably, the transition metal is selected from ruthenium(Ru), nickel (Ni), platinum (Pt), iridium (Ir), copper (Cu), cobalt(Co), iron (Fe), palladium (Pd), and rhodium (Rh), with Ru beingpreferred. The surface lattice parameter of the transition metal ispreferably matched to that of graphene, having a lattice mismatch of≦15%, or is such that higher order commensurate or incommensurateinterface structure develops that still provides a good structuraltemplate for graphene growth. The growth surface is not limited to aparticular crystallographic plane or surface structure, but preferablyexhibits a hexagonal crystal structure, thereby providing a template forgraphene growth. Ruthenium (0001) is the most preferred, while theplatinum, copper, nickel and iridium (111) faces are also useful. Themetal surface preferably consists of atomically smooth terracesalternating with atomic surface steps, so as to permit the facilenucleation and growth of graphene layers followed by growth via Cincorporation along the edges of the graphene layer.

The growth process is continuous, such that the graphene layerpropagates across terraces and over step edges in the “downhill”direction during growth. Additional C layers may nucleate and grow ontop of or beneath the first and/or subsequent layers to produce aplurality of graphene layers sequentially stacked one on top of theother. Once the metal substrate is selected, the surface can beinitially cleaned, for example, by repeated cycles of Ar⁺ ionbombardment and high-temperature annealing in an ultrahigh vacuum (UHV)or high vacuum (HV) process chamber. The growth process furtherencompasses heating the metal substrate to temperatures between 500° C.and 2000° C. depending on the metal selected, preferably to 700° C. to1500° C. for several seconds to several minutes and then slowly, e.g.,at a rate ≦10° C.-50° C. per minute, cooling to 300° C. to 1000° C.,preferably 600° C. to 900° C. while exposing the metal surface to acarbon source, e.g., ethylene.

As the metal surface cools, graphene nucleates at random sites on thesurface and the size of the graphene domain increases gradually withdecreasing temperature as C atoms are continually incorporated along theedges of the graphene layer. This results in graphene domains withlinear dimensions preferably in excess of 200 μm. A detailed descriptionof this process with particular application to Ru(0001) is described inSutter 2008.

Specifically, if the transition metal substrate is ruthenium (Ru) andthe growth plane is the Ru(0001) crystal surface, the Ru(0001) surfaceis initially cleaned by repeated cycles of alternating oxygen adsorptionand high-temperature annealing in an ultrahigh vacuum (UHV) or highvacuum (HV) process chamber, or by longer oxygen exposure at 550° C. to950° C., followed by oxygen adsorption and flash annealing. This isfollowed by heating to 950° C. to 1250° C. for several seconds toseveral minutes while exposing the Ru(0001) to a carbon source, e.g.,ethylene (so as to enrich the Ru crystal with interstitial carbon), andthen slowly (at a rate ≦20° C. per minute) cooling to 700° C. to 900° C.As the Ru(0001) surface cools, graphene nucleates at random sites on thesurface and the size of the graphene domain increases gradually withdecreasing temperature as C atoms are continually incorporated along theedges of the graphene layer.

In an alternative, a surface template for graphene growth may beprovided by a suitable transition metal or alloy thin film formed on asupporting substrate. The substrate is not limited to any particularmaterial, but must be able to support the transition metal or alloy.That is, the underlying substrate must have physical and chemicalproperties which facilitate the formation of a suitable transition metalor alloy overlayer which then serves as a surface template for graphenegrowth. An example is Ru on SiO₂ on silicon. (See Sutter, E. A., et al.,“Graphene growth on polycrystalline Ru thin films,” Appl. Phys. Lett.,95, p. 133109 (2009), which is incorporated by reference herein in itsentirety.) Another example is Ru on sapphire (Al₂O₃(0001)), whichprovides particularly well-ordered Ru(0001) template surfaces forhigh-quality graphene growth (See Sutter, P. W., et al., “Graphenegrowth on epitaxial Ru thin films on sapphire,” Appl. Phys. Lett., 97,p. 213101 (2010), which is incorporated by reference herein in itsentirety.)

In some embodiments, the substrate and/or the transition metal or alloyfilm may deviate from planarity. In some cases, this deviation may be acurvature whose radius is of the order of, or greater than, that of thelateral dimensions of the graphene domains. In other cases, thesubstrate may exhibit curvature whose radius is significantly smallerthan the lateral dimensions of the graphene domains. The substratecurvature may have a radius on the order of 100 μm, or greater or lessthan that depending on the particular application. An example is a Ruthin film on a patterned, non-planar fused silica substrate. (SeeSutter, E. A., et al., “Monolayer graphene as ultimate chemicalpassivation layer for arbitrarily shaped metal surfaces”, Carbon 48, p.4414 (2010), which is incorporated by reference herein in its entirety.)

The method of processing the monolayer graphene further includes stepsof exposing the partially-covered surface of the metal, e.g., Ru(0001),Ir(111), Ni(111), Pt(111), or Cu(111), to an ambient gas, e.g., oxygen(O₂) or nitrogen oxide (NO₂), and tuning the graphene-metal interactionby interfacial reaction of the ambient gas with the surface of the metalbeneath the graphene sheet. In particular, the step of exposing can beperformed by heating partially graphene-covered metal in an ambient gas,e.g., O₂/Argon, flow for a fixed period of time, e.g., 1-24 h. Duringthis step the ambient gas either intercalates beneath the graphene layeror etches the graphene layer depending on the conditions of theexposure. Thus, the step of tuning the graphene-metal interface isachieved by regulating the surface exposure. The ambient gas moleculesintercalate on the surface of the metal at temperatures below 400° C.,preferably between 200° C. and 380° C., which in turn decouples thegraphene from the metal. Under these conditions, the ambient gas ispreferably a diatomic molecule, such as oxygen, or a molecule thatbreaks down into a diatomic molecule, e.g., NO₂→NO(+½O), that does notetch graphene but selectively adsorbs on the metal surface beneath thegraphene sheet as illustrated in FIGS. 5A-5C. The complete intercalationof macroscopic domains that are tens of micrometers in size decouplesthe graphene and restores the linear π bands of its electronicstructure. The graphene sheet's presence affects the metal-adsorbateinteraction. The intercalation can be reversed and etching increased byraising a temperature above 400° C., although below the meltingtemperature of the transition metal, e.g., Ru˜2334° C., Ni˜1455° C.,preferably between 400° C. and 800° C. Under elevated temperature, O₂exposure, for example, causes the preferential etching of graphene pointdefects and edges. These effects become much more pronounced forgraphene on metals that facilitate the dissociation of O₂, releasinghighly reactive oxygen atoms (see FIGS. 2A and 2B).

In another embodiment, the intercalation of species (atoms, molecules)between graphene and metal, and the resulting decoupling of the graphenesheet from the metal, can be accomplished in a liquid solutionenvironment, including basic aqueous solutions of potassium hydroxide(KOH) or sodium hydroxide (NaOH) with typical concentrations between 0.1molar and 4.0 molar. During the process of intercalation the sample isthe working electrode of an electrochemical cell with a suitable (forexample Pt) counter electrode, and with an applied working electrodepotential between −0.1 V and −10 V relative to the counter electrode. Inthis process, the graphene surface is typically covered by a polymerlayer (for example, poly(methyl methacrylate), PMMA, or a similarpolymer that is non-soluble in aqueous environments) that acts as amechanical support layer and protective surface layer for the decoupledgraphene.

In another aspect, the interface of graphene with metals ormetal-decorated substrates, such as Ru(0001), is conducive to controlledchemical reactions based on novel functionality of the confined spacebetween a metal surface and a graphene sheet. In one embodiment, theconfined space measures about 3.3 Å from the surface of the metalsubstrate to the graphene layer. This approach contrasts with thelong-held notion that graphitic carbon acts as a poison that suppressesdesired chemical reactions in surface chemistry and catalysis. However,in the preferred embodiment, the graphene sheet does not merely act as apassive spectator but it provides two types of novel functionality. Itgenerates an extended confined space that can give rise to significantsteric hindrance, which should preclude the access of large species andmay control the orientation of small molecules. In addition, similar toother strategies, e.g., coadsorption, the presence of the graphene sheetcan affect important reaction parameters, such as adsorption energies ofmolecules adsorbed on the metal beneath graphene. Chemistry at theinterface between graphene and the transition metal thus represents anew approach for tuning chemical reactions on transition-metal surfaces.

FIG. 10 is a schematic illustration of how the limited distance betweena graphene sheet and its substrate, generally 1.8 to 4.0 Å, hindersentry of large molecules while permitting small molecular and atomicspecies to intercalate. By this means the graphene sheet provides sizecontrol for interfacial reactions. FIG. 11 is a schematic illustrationof materials synthesis at the interface between graphene and itssubstrate. The reactant species intercalate between the graphene layerand its substrate, where they react to form a new material. This processcould be used, for example, to produce a gate dielectric between thegraphene and its substrate, thus isolating graphene electrically fromthe metal. The metal can then serve as a gate electrode, e.g., in afield-effect device.

While the processing of graphene by reactive intercalation has beendescribed in connection with what is presently considered to be the mostpractical and preferred embodiments, it is to be understood that theinvention is not to be limited to the disclosed embodiments, but on thecontrary, is intended to cover various modifications and equivalentarrangements included within the spirit and scope of the appendedclaims.

EXAMPLES

The examples set forth below also serve to provide further appreciationof the invention but are not meant in any way to restrict the scope ofthe described invention.

Example 1

Graphene epitaxy was performed in ultrahigh vacuum (UHV) by carbonsegregation from a Ru(0001) single crystal pre-exposed to ethylene at atemperature of 1,150° C., as described in Sutter 2008. Specifically,graphene growth was carried out by thermal cycling of a Ru(0001) singlecrystal in UHV to achieve the controlled layer-by-layer growth of largegraphene domains on Ru(0001). At high temperature, C was absorbed intothe Ru bulk. Slow cooling from 1,150° C. to 825° C. lowered theinterstitial C solubility by a factor of 6, driving significant amountsof C to the surface. The result was an array of lens-shaped islands ofmacroscopic size (>100 μm) covering the entire Ru(0001) substrate, asshown in FIG. 7A, which is a LEED pattern (155 eV) of the as-grownmonolayer graphene on Ru(0001).

Example 2

Graphene growth and intercalation were observed in real time bybright-field low-energy electron microscopy (LEEM), using an ElmitecLEEM V field emission microscope.

FIG. 2A is a sequence of real-time LEEM images of epitaxial monolayergraphene on Ru(0001) obtained during O₂ exposure above 450° C., showingoxygen etching of a graphene domain (P=5×10⁻⁷ Torr; T=550° C.). At hightemperatures, O₂ exposure causes the preferential etching of graphene atpoint defects and edges. These effects became much more pronounced forgraphene on metals that facilitate the dissociation of O₂, releasinghighly reactive oxygen atoms. As illustrated in FIGS. 2A and 2B, aninitial drop in image intensity within areas of exposed metal thataccompanies the adsorption of oxygen on the metal surface is followed byrapid etching of the graphene edge. The resulting reverse edge-flowcontinues until no detectable graphene remains on the surface.

Example 3

FIG. 2C is a sequence of LEEM images of epitaxial monolayer graphene onRu(0001) obtained during O₂ exposure below 400° C., giving rise tooxygen intercalation and selective oxidation of the Ru surface beneathgraphene. As illustrated in FIGS. 2C and 2D, when similar graphenedomains as described in Example 2 are exposed to O₂ at lowertemperatures, the initial oxygen adsorption on the exposed metal isagain followed by changes in image contrast that begin near the edge andextend progressively toward the center of the graphene domain.Throughout this process, however, the modified graphene sheet remainsclearly distinguishable from the surrounding metal surface.

While oxygen intercalation during O₂ exposure of graphene on Ru(0001)has been postulated in Zhang, H. et al. (J. Phys. Chem. C 2009, 113,8296, incorporated herein by reference in its entirety) on the basis ofsmall-scale scanning tunneling microscopy (STM) combined withphotoelectron spectroscopy, the LEEM images in FIGS. 2C and 2Dillustrate that such intercalation is readily scaled up to modify thegraphene-Ru interface over macroscopic areas. The front between as-grownand modified graphene is sharply delineated throughout this process, andhigh-resolution STM shows it to be abrupt on the atomic scale asillustrated in a low-bias scanning tunneling microscopy image of theboundary between as-grown and oxygen intercalated graphene on Ru(0001)shown in FIG. 6. As illustrated in FIG. 2C, for the lens-shapedmonolayer graphene domains on Ru, the intercalation proceeds readilyfrom the straight edge and across substrate steps in the downhilldirection but is often hindered at the opposite (rounded) edge of thedomain.

Furthermore, FIG. 7A is a LEED pattern (155 eV) of as-grown monolayergraphene on Ru(0001) and FIG. 7C illustrates the same but with oxygen(O₂) intercalated beneath monolayer graphene on Ru(0001). Accordingly,FIGS. 7B and 7D illustrate that electron microdiffraction on either sideof the intercalation front has a transition from the well-knowngraphene-Ru(0001) Moiré to a structure with additional half-integerdiffraction spots, identified as an ordered p(2×1) adlayer phase with0.5 monolayer (ML) of oxygen chemisorbed on the Ru surface beneath thegraphene sheet.

Example 4

Exposure to a different oxygen precursor, NO₂, at the same temperature,as provided in Examples 3, induces similar behavior, namely, theselective modification of the epitaxial graphene monolayer byintercalation. FIG. 2E is a sequence of LEEM images of epitaxialmonolayer graphene on Ru(0001) obtained during NO₂ exposure below 400°C., giving rise to nitrogen monoxide intercalation and selectiveoxidation of the Ru surface beneath graphene. As illustrated in FIGS.2C-2F, overall, the intercalation by exposure to NO₂ proceedssubstantially faster than that from O₂. It advances uniformly from alledges of the graphene domain. In contrast to the case of O₂, theintercalation front is only initially abrupt and then becomesprogressively more diffuse as it propagates from the edge toward thecenter of the domain.

Example 5

Measurements of the projected band structure provide direct evidence ofthe dramatic change in the interfacial coupling between graphene andmetal caused by the processes shown in FIGS. 2C-2F. FIGS. 3A and 3C aremicrometer-sized spot angle-resolved photoelectron spectroscopy(micro-ARPES) maps of the band structure of as-grown monolayer grapheneon Ru(0001) before and after O₂ exposure, showing the restoration oflinear π bands crossing the Fermi energy and hole doping with acharge-neutrality point 0.5 eV above the Fermi energy, E_(F). Asillustrated in FIG. 3A, for as-grown monolayer graphene on Ru(0001),metal d states hybridize with the occupied graphene π orbitals. Thisstrong electronic interaction is reflected by a pronounced (2 eV)downward shift of the π bands and the opening of a gap between the π andπ* states near E_(F). In contrast, O₂ (or NO₂) exposure at temperaturesof 300° C. fundamentally alters the electronic band structureillustrated in FIG. 3C. In the modified graphene domains, the π-dhybridization is lifted (see FIGS. 3B and 3D), leading to the appearanceof well-defined graphene π bands crossing the Fermi level with linearband dispersion at the (K, K′) points of the Brillouin zone. Theobserved intense π bands and the weaker σ bands closely match the bandstructure of free-standing graphene.

Example 6

Charge transfer shifts the neutrality point (“Dirac point”) to 0.5 eVabove E_(F), thereby inducing a net hole doping of the graphene sheet.The oxygen exposure also affects the (0001) projected band structure ofRu, notably at the zone center, where the occupied band at −2 eV isstrongly modified, consistent with O chemisorption on the metal surfacebeneath the graphene sheet. The formation of a strongly bound, orderedoxygen adlayer structure causes the coupling of Ru 4d with O 2p states.This saturates the metal d states and weakens the interaction withgraphene, which is now limited to residual electron transfer from thegraphene sheet to the strong acceptors at the metal surface. The STMcontrast changes across the intercalation boundary, a sharp transitionfrom a strongly corrugated moiré to a planar sheet with honeycombstructure similar to that found for free-standing graphene, as shown inFIG. 6.

Example 7

Additional experiments were performed to address the kinetics of oxygenintercalation and graphene etching as well as the reaction mechanism forRu oxidation beneath monolayer graphene. Surface structure of thegraphene-Ru was determined in situ by low-energy diffraction (LEED) andIV-LEED in the same system. Temperature dependent graphene intercalationand etching rates were extracted from the motion of the intercalationfront. Band structure [E(k_(x),k_(y))] maps on as-grown and intercalatedgraphene were obtained at room temperature in-situ in an energy-filteredLEEM III instrument by collecting angle resolved photoelectron spectrafrom micrometer-sized sample areas (micro-ARPES). Synchrotronultraviolet radiation (National Synchrotron Light Source beamline U5UA;photon energy hv=42 eV) incident normal to the sample was used to excitephotoelectrons, which were energy filtered by an imaging energy analyzer(energy resolution <0.3 eV), and whose angular distribution was mappedin reciprocal space using the electron optics and detector system of themicroscope. Scanning tunneling microscopy (STM) of the grapheneintercalation edge was performed at room temperature in situ in aseparate UHV system, using the procedures outlined above for graphenegrowth and oxygen intercalation. UHV-SEM imaging and nano-Auger electronspectroscopy were performed in a commercial system (OmicronNanotechnology) equipped with a field-emission SEM and Auger electronanalyzer, using the focused SEM electron beam (energy: 3 keV; current:100 pA) to excite Auger electrons.

Real-time LEEM observations during O₂ exposure at different temperatureswere used to analyze the competition between intercalation (leading tothe selective oxidation of Ru beneath the graphene sheet) and etching ofgraphene. The results are summarized in FIGS. 4A-4B, showing that thetwo processes are thermally activated but follow distinctly differentArrhenius relations.

The overall reaction rates can be written as

R=fAexp(−E _(A) /k _(B) T),

where A is the attempt frequency of the rate-determining step, f is an“efficiency factor”involving the abundance of the reactant (O₂), andE_(A) and k_(B)T denote the activation barrier and thermal energy,respectively. A fit of this relation to the measured reaction ratesgives E_(A) and the prefactor, fA. For oxygen intercalation,E_(A)=0.38±0.05 eV (see FIG. 4A). A small prefactor, fA=10¹⁰ s⁻¹,indicates a low concentration of mobile species arriving at the reactionfront. Oxygen etching of the graphene domain involves a largeractivation energy, E_(A)=1.1±0.1 eV, so it should generally proceed witha lower rate than intercalation. However, the prefactor for oxidativeattack (3×10¹⁵ s⁻¹) is much larger than for oxygen intercalation,reflecting the unrestricted access of reactants (O, O₂) from the exposedmetal to the graphene edge. The overall result of these complex reactionkinetics is a competition between the two processes: intercalationdominates at low temperatures, and a transition to etching occurs forhigher temperatures (see FIG. 4B).

Example 8

The observed partitioning into two distinct regimes shown in Example 7suggests that intercalated graphene should remain stable to temperaturesof at least 400° C. Real-time LEEM during annealing shown in FIG. 9A canthus be used to explore the stability of the interfacial oxygen layerand the reversibility of the intercalation process. As illustrated inFIG. 9C, heating from the intercalation temperature to 400° C. causes nochanges in the contrast of the free Ru surface, consistent with a high Obinding energy. The contrast of the intercalated graphene domain, on theother hand, changes progressively above an onset temperature of 380° C.,reverting from the dark contrast of an intercalated domain to thecharacteristic bright appearance of as-grown monolayer graphene. On thebasis of these observations, it is concluded that oxygen intercalationis reversible. The presence of graphene affects the binding of oxygen onRu(0001), weakening the coupling so desorption can occur at temperaturesat which O remains strongly bound on the free metal surface.

A comparison of the effects of two different oxygen-carrying precursors,O₂ and NO₂, is an important element to shed light on the mechanism ofselective Ru oxidation beneath graphene at low temperatures. O₂adsorption on bare Ru(0001) is dissociative, initially with a stickingcoefficient near unity. At the O₂ pressures used in Examples 1-8, itgives rise to a progression of ordered O-adlayer structures, terminatingin a p(2×1)-O structure at 0.5 ML coverage. At this point, the O₂sticking coefficient drops sharply, causing an apparent saturation ofadsorption. Higher doses do not lead to the continued release of Oatoms, but the “excess” O₂ simply desorbs. NO₂ adsorption at elevatedtemperatures, on the other hand, involves the dissociation to atomicoxygen and NO. The chemisorbed O again forms ordered adlayers, albeit tocoverages up to 1 ML. NO desorbs from the free Ru surface at thetemperatures considered here. For Ru(0001) partially covered bymonolayer graphene, O₂ exposure at elevated temperatures leads todissociative adsorption of oxygen on the exposed Ru surface but not onthe graphene. Adsorbed O atoms diffuse on Ru(0001), so they can reachthe graphene edge and start to decouple the graphene from the metalsurface. This process of O₂ dissociation on free Ru and intercalation byO diffusion into areas beneath the graphene domain could in principlecontinue until the entire graphene sheet is decoupled. If this is thecase, the kinetics of O-diffusion on graphene-covered Ru must differsubstantially from that on free Ru(0001).

The Arrhenius analysis, provided in Example 7, showed that for grapheneintercalation, the activation energy for the reaction-limiting step isE_(A)=0.38 eV, which is substantially lower than the measured andcalculated O diffusion barrier on Ru (0.5-0.7 eV). The atomically abruptintercalation front suggests that the limiting step occurs at the frontitself and thus is the decoupling of carbon from the metal. Hence, thediffusion of the intercalating species to the reaction front cannot bethe limiting step but must be fast with an activation energy below 0.38eV. The de-intercalation experiments indeed show that the presence ofgraphene weakens the binding of chemisorbed O on Ru(0001), which meansthat it could similarly reduce the activation energy for O diffusion atthe graphene-Ru interface since the diffusion barrier on transitionmetals scales linearly with adsorbate binding energy.

Without being bound by theory, a second possible scenario that mayexplain the facile oxygen transport between monolayer graphene and Ru isthe interfacial diffusion that could involve a mobile species differentfrom chemisorbed O. Molecular O₂, which is weakly bound to the metal,can be expected to diffuse laterally without significant activationbarriers (see FIG. 5C). While on the free Ru surface O₂ eitherdissociates or desorbs (see FIGS. 5A and 5B), in the presence of apartially detached graphene sheet that is itself impenetrable to oxygenmolecules, the possibility arises that O₂ molecules populate the spacebetween Ru and graphene, diffuse to the reaction front, and dissociatethere to drive the continued oxidation of the Ru surface and decouplingof the graphene sheet as illustrated in FIG. 5C.

The suggested diffusion of O₂ between the decoupled graphene and theadjacent metal implies that a broader range of chemical reactionsinvolving small molecules could be performed in the confined spacebetween graphene and a metal surface. Comparing the O van der Waalsradius (1.52 Å) and the O₂ bond length (1.21 Å) with the graphene-metalspacing (3.3 Å, typical for weakly coupled graphene on metal) indicatesthat molecular intercalation is indeed plausible.

To further corroborate the possibility of intercalation by diatomicmolecules, the intercalation by NO₂ exposure was considered. Followingthe initial exposure to NO₂, which causes O adsorption and startsdecoupling of the graphene, it again becomes possible for NO moleculesto be trapped between graphene and the metal. The activation energy forNO diffusion on Ru(0001) (0.16 eV) is significantly lower than those ofthe other possible dissociation products (N, 0.94 eV; 0, 0.5-0.7 eV), sotrapped NO could rapidly diffuse to the intercalation front and maybecome the active species controlling the subsequent decoupling of thegraphene sheet. Without being bound by theory, it is anticipated thatthe presence of nitrogen beneath the graphene sheet would serve as afingerprint corroborating molecular intercalation. To detect possible Nspecies, ultrahigh-vacuum scanning electron microscopy (UHV-SEM) coupledwith nano-Auger electron spectroscopy (nano-AES) was performed and issummarized in FIGS. 8A-8C. UHV-SEM clearly identified the monolayergraphene domains by their characteristic lens shape. While as-growngraphene has a uniform UHV-SEM contrast, graphene domains intercalatedfrom NO₂ show a dark rim surrounding a bright central area. Nano-AESdetected oxygen (O_(KLL)) in both regions. There was no detectableN_(KLL) signal in the darker boundary region, but the central brighterarea gave rise to additional N_(KLL) lines. Both the core-shellstructure of the intercalated graphene domains and the presence of N inthe central region are consistent with the intercalation behavior shownin FIGS. 2E-2F and the suggested scenario of a transition from atomic Oto molecular NO intercalation during NO₂ exposure. Diatomic moleculessuch as O₂ or NO can therefore populate the space between weakly coupledgraphene and metal and as rapidly diffusing species contribute to thecontinued decoupling of the graphene sheet as illustrated in FIGS.5A-5C.

All publications and patents mentioned in the above specification areherein incorporated by reference in their entireties. Variousmodifications and variations of the described materials and methods willbe apparent to those skilled in the art without departing from the scopeand spirit of the invention. Although the disclosure has been describedin connection with specific preferred embodiments, it should beunderstood that the invention as claimed should not be unduly limited tosuch specific embodiments. Indeed, those skilled in the art willrecognize, or be able to ascertain using the teaching herein and no morethan routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

1. A method of processing graphene, the method comprising, providing agraphene layer in which graphene interacts with a surface of a metalsubstrate under the graphene layer; exposing the surface of the metalsubstrate to an ambient gas; and tuning the graphene-substrateinteraction by interfacial reaction of the ambient gas on the surface ofthe metal substrate.
 2. The method of claim 1, wherein the metalsubstrate is selected from the group consisting of ruthenium (Ru),nickel (Ni), platinum (Pt), iridium (Ir), copper (Cu), cobalt (Co), iron(Fe), Palladium (Pd), and rhodium (Rh).
 3. The method of claim 1,wherein the ambient gas is selected from the group consisting of oxygen(O₂), nitrogen oxide (NO₂), nitrogen (N₂), hydrogen (H₂), chlorine (Cl),fluorine (F), bromine (Br), iodine (I), ammonia (NH₃).
 4. The method ofclaim 1, wherein tuning the graphene-substrate interaction comprisesintercalating the ambient gas between the graphene layer and the surfaceof the metal substrate at temperatures below 400° C.; and decoupling thegraphene from the metal substrate as a result of the ambient gasintercalation.
 5. The method of claim 4, further comprising: reversingthe intercalation of the ambient gas between the graphene layer and thesurface of the metal substrate by raising the metal substrate to atemperature above 400° C.
 6. The method of claim 4, wherein theintercalation under the ambient gas on the surface of the metalsubstrate occurs at temperatures between about 20° C. and 400° C.
 7. Themethod of claim 5, wherein reversing the intercalation occurs betweenabout 400° C. and the melting point of the metal substrate.
 8. Themethod of claim 4, wherein the decoupling of the graphene isaccomplished by a selective oxidation of the metal surface.
 9. Themethod of claim 4, wherein the decoupling of the graphene isaccomplished by nitridation, hydrogenation, or reaction with Cl, F, Br,or I.
 10. The method of claim 4, wherein the ambient gas is an atomicgas, a gas of diatomic or larger molecules, or a gas of molecules thatbreak down into atoms or smaller (diatomic or larger) molecules betweenthe graphene layer and the metal substrate surface.
 11. The method ofclaim 4, wherein the ambient gas is nitrogen oxide (NO₂).
 12. The methodof claim 11, wherein the nitrogen oxide (NO₂) forms a diatomic NO gas ina space between the graphene layer and the metal substrate.
 13. Themethod of claim 1, wherein tuning the graphene-substrate interactioncomprises restoring a characteristic Dirac behavior of isolatedgraphene, wherein graphene is monolayer, or has related charge-carriercharacteristics of bilayer graphene, few-layer graphene or multilayergraphene.
 14. The method of claim 1, wherein tuning thegraphene-substrate interaction comprises electrically isolating thegraphene layer from the metal substrate.
 15. The method of claim 14,wherein electrically isolating the graphene layer from the metalsubstrate comprises forming a dielectric between the graphene layer andthe metal substrate.
 16. A method of processing graphene via interfacialreactions, the method comprising, providing a graphene layer in whichgraphene interacts with a surface of a metal substrate under thegraphene layer; exposing the surface of the metal substrate to anambient gas, the metal substrate selected from the group consisting ofruthenium (Ru), nickel (Ni), platinum (Pt), iridium (Ir), copper (Cu),cobalt (Co), iron (Fe), palladium (Pd), and rhodium (Rh) and the ambientgas selected from the group consisting of oxygen (O₂), nitrogen oxide(NO₂), nitrogen (N₂), hydrogen (H₂), chlorine (Cl), fluorine (F),bromine (Br), iodine (I), and ammonia (NH₃); and tuning thegraphene-substrate interaction by interfacial reaction of the ambientgas on the surface of the metal surface, wherein tuning thegraphene-substrate interaction comprises: intercalating the ambient gasbetween the graphene layer and the surface of the metal substrate attemperatures below 400° C.; and modifying the graphene-substrateinteraction or decoupling the graphene from the metal substrate as aresult of the ambient gas intercalation.
 17. The method of claim 16,further comprising: reversing the intercalation of the ambient gasbetween the graphene layer and the surface of the metal substrate byraising the metal substrate to a temperature above 400° C.
 18. A methodof performing chemical reactions at an interface between graphene and ametal, the method comprising, exposing a surface of a metal substrateunder a graphene layer to chemical species chosen from the groupconsisting of atomic species, molecular species, and a combinationthereof; performing chemical reactions of the chemical species with eachother or with the surface of the metal substrate in a space beneath thegraphene layer by intercalation, and regulating intercalation by varyingreaction parameters of partial pressures of all chemical species, sampletemperature, and sequence of exposure to different reactant species,which can be simultaneous or sequential with different waiting andexposure times.
 19. The method of claim 18, wherein the graphene layerprovides steric hindrance, limits access of atomic and molecular speciesbased on size, and controls orientation of the atomic and molecularspecies.
 20. The method of claim 18, wherein the metal substrate is atransition metal.
 21. The method of claim 20, wherein the transitionmetal is selected from the group consisting of ruthenium (Ru), nickel(Ni), platinum (Pt), iridium (Ir), copper (Cu), cobalt (Co), iron (Fe),palladium (Pd), and rhodium (Rh).
 22. The method of claim 18, whereinthe molecular species are selected from the group consisting of oxygen(O₂), nitrogen oxide (NO₂), nitrogen (N₂), hydrogen (H₂), chlorine (Cl),fluorine (F), bromine (Br), iodine (I), and ammonia (NH₃).
 23. Themethod of claim 18, wherein the atomic species are selected from thegroup consisting of silicon (Si), boron (B), aluminum (Al), zinc (Zn),chromium (Cr), titanium (Ti), zirconium (Zr), hafnium (Hf), scandium(Sc), yttrium (Y), magnesium (Mg), calcium (Ca), strontium (Sr), barium(Ba), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sa), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), andytterbium (Yb).
 24. The method of claim 18, wherein regulatingintercalation comprises: allowing the intercalation of the atomic ormolecular species on the surface of the metal substrate at temperaturesbelow 400° C.; decoupling the graphene from the metal substrate as aresult of the intercalation; and reversing the intercalation of theatomic or molecular species on the surface of the metal substrate byraising the temperature above 400° C.
 25. The method of claim 18,wherein the space beneath the layer of graphene has a height between 1.8Å and 4 Å.